In general, tires have the following local defects due to the manufacturing process: low-frequency warp called runout and a shape having a slope larger than that of runout called bulge/dent. Accordingly, the shape of a surface to be measured of a tire is measured. Thereafter, the runout and bulge/dent are evaluated on the basis of obtained shape data, and it is determined whether the tire is defective.
In addition, due to the manufacturing process, the tires have a portion called burr or spew which has a width smaller than that of a bulge/dent and a height larger than that of the bulge/dent. If the shape data includes the shape of a burr or a spew, it is difficult to accurately evaluate the runout and the bulge/dent from the shape data.
Accordingly, PTL 1 describes a technique for calculating radial runout evaluation data to evaluate the radial runout by scanning a tread surface in the circumferential direction at a predetermined sampling period using a non-contact displacement meter to obtain height data for one round and removing a noise component from the height data. At that time, according to PTL 1, if the difference between the median of the height data items at a plurality of consecutive sample points, at the center of which there is a given sample point, and each of the height data items is greater than a threshold value, the height data item is determined to be a noise component.
However, in PTL 1, a value that is slightly greater than a normal variable value of the runout is used as the threshold value to extract the noise component. Accordingly, a bulge/dent and a normal runout that exceeds the normal variable value may be determined to be noise components and may be removed, which is problematic. In addition, according to PTL 1, a height data item that differs from the median by the threshold value or greater is simply determined to be a noise component and is removed without taking into account the number of consecutive height data items that differ from the median by the threshold value or greater. Accordingly, a height data item indicating a bulge/dent having a height that is substantially the same as the height of the noise component may be removed.